This invention relates to magnetometers and magnetic field strength measurement methods, particularly for use in an electronic compass.
In navigation and vehicle tracking operations it is important to be able to determine directions with a high degree of accuracy. It is also important to be able to provide directional data to electronic systems which are employed to accomplish guidance and tracking computations, particularly where the vehicle involved is moving relatively rapidly so that manual processing of the directional data for navigation or tracking is impractical. This is often accomplished using an electronic magnetometer which measures the magnitude of the earth's magnetic field. Ordinarily, the field strength is measured in two different directions, typically 90 degrees apart, so that the orientation of the magnetometer can be computed trigonometrically from the results of those measurements, thereby providing an electronic compass.
One way of measuring magnetic field strength electronically is through the use of Hall effect devices. When placed in a magnetic field, such devices provide a voltage output related to the strength of the magnetic field. While Hall effect devices are generally low in cost and readily available, most of them are not sensitive enough to be used for an electronic compass. Those which may be sensitive enough to use in an electronic compass are relatively expensive.
Another way of measuring magnetic field strength electronically is to use a magnetic core based on the flux gate theory. In this technique, a core of magnetic material is magnetically coupled to a sense coil, which is wound around the core, and the core is periodically driven into saturation by a control field applied by a coil which is typically separate from the sense coil. When the magnetic material is not saturated, its permeability is high, so that the core flux density of any external magnetic field in which the core is placed is also high. However, when the core is saturated by the control field, its permeability is low, so that the external magnetic field is not drawn into the core and the core flux density of any such field is low. The change in flux density when the core is driven into saturation and back out induces an electromotive force in the sense coil which, when the coil is connected to a load, produces a current through the coil related to the strength of the external magnetic field. In magnetometers heretofore known, the induced voltage is measured and used to produce a signal representative of the strength of the external magnetic field. Such flux gate devices provide the necessary sensitivity for an electronic compass and are less expensive than comparable Hall effect devices.
One of the problems encountered in designing an accurate flux gate compass is that the voltage produced by the sense coil is a function not only of the strength of the external magnetic field, but also the magnetization of the magnetic material, i.e., the relationship of the magnetic flux density to the magnetic field strength in the core material, which is non-linear and relatively unpredictable from core to core. This has typically been overcome by using feedback from the voltage generated by the sense coil to produce and apply to the core a magnetic field that cancels out the external magnetic field so that the core characteristics are not a factor. The current needed to produce the cancellation field is indicative of the strength of the external magnetic field.
However, there are other problems encountered in the design of an accurate flux gate compass that have heretofore not been overcome. One such problem is that by measuring the voltage produced by a sense coil it is difficult to achieve a high degree of accuracy. This is because the voltage produced by the coil is dependent not only on the external magnetic field strength, but also on the rate of change of the magnetic field and the magnetization properties of the core. That the voltage is dependent on the rate of change of magnetic flux in the core makes voltage measurement very sensitive to changes in the frequency of the signal that drives the control field.
Another problem is that the time needed to switch the core from the unsaturated state to the saturated state and back reduces the accuracy with which the signal produced by the sense coil can be converted to a signal representative of the strength of the external magnetic field.
A further problem with prior flux gate magnetometers is that saturation of the core leads to excessive current flow in the drive coil, producing power consumption and dissipation problems.
Flux gate magnetometers are described, for example, in Mach et al. U.S. Pat. No. 4,305,035, entitled "Magnetic Field Amplitude Detection Sensor Apparatus", in Rhodes U.S. Pat. No. 4,300,095, entitled "Self Excited Field Saturable Core Magnetic Field Detection Apparatus", in Rhodes U.S. Pat. No. 4,290,018, entitled "Magnetic Field Strength Measuring Apparatus With Triangular Waveform Drive Means", and in Mound et al. U.S. Pat. No. 3,605,011, entitled "Control Apparatus". However, all of these devices measure the voltage produced by the sense coil, and all appear to provide no solution to the problems of core characteristic variability, switching speed and power consumption and dissipation.
Thence, there is a need for an electronic magnetometer and magnetic field strength measurement method that is more tolerant of component variability than those heretofore available and which reduces power consumption and dissipation problems.